(i) Field of the Invention
This invention relates to a process for recovering nickel and cobalt from nickel- and cobalt-containing laterite ores and, more particularly, relates to a process for producing an enriched nickel and cobalt sulphide intermediate from nickel-and cobalt-containing laterite ores, and for producing nickel and cobalt metal from the nickel and cobalt sulphide intermediate.
(ii) Description of the Related Art
Sulphuric acid leaching of laterite in pressure vessels has been practiced commercially since 1959 at Moa Bay in Cuba, as described by Chalkey and Toirac in a paper entitled “The acid pressure leach process for nickel and cobalt laterite. Part I: Review of operations at Moa” presented January, 1997. The original process plant includes ore leaching, washing of the ore leach discharge slurry in a counter-current decantation wash circuit (CCD wash circuit), reduction of the wash circuit product solution by contact with hydrogen sulphide, neutralization of the acid in the reduced solution with limestone mud, washing of the thickened neutralization solids in the CCD wash circuit, and recovery of nickel and cobalt as mixed sulphide by contact of the neutralized solution with hydrogen sulphide.
Neutralization of the solution prior to sulphide precipitation is required so that high recoveries of nickel and cobalt may be obtained in sulphide precipitation. The neutralization circuit is operated at pH 2.1 to 2.3, which is sufficiently high that metals recoveries are high in sulphide precipitation. However, the neutralized solution contains significant concentrations of silicon and iron at this relatively low pH, resulting in contamination of the mixed sulphide with these elements. This in turn limits the options for downstream processing of the mixed sulphide.
Sulphide precipitation is carried out at relatively high temperature and pressure in this operation, which results in a large recirculating load of hydrogen sulphide, which is expensive to recover and reuse. The sulphide precipitation step, when operated at the relatively high temperature and pressure, is sensitive to the presence of copper, which results in mixed sulphide with poor liquid-solid separation characteristics. Hence the need to precipitate copper in the reduction step prior to neutralization. The presence of copper in the neutralization solids places restrictions on how these solids can be washed in the CCD wash circuit, since the copper redissolves in this circuit if the solids are combined with slurry containing sufficient acid and ferric iron. This dictates the point of entry of the neutralization solids to the multi-stage wash circuit and limits the recovery of nickel and cobalt from the solution entrained with the neutralization solids. The use of process water in the wash circuit increases the fresh water load on the process plant.
The second laterite processing plant constructed to produce mixed sulphide intermediate from pressure acid leach liquor is situated at Murrin Murrin in Australia. The operation of this plant is described in a paper entitled “Murrin Murrin Nickel-Cobalt Project”0 presented by Motteram et al. May 31, 1996 and described in a paper entitled “Murrin Murrin CCD1 rake mechanism modification: the decision and the result” by Hewitt et al. Improvements to the process incorporated into the Murrin Murrin plant include a preneutralization of a portion of the ore leach discharge slurry (slurry neutralization) before liquid/solid separation in a CCD wash circuit. This limits the concentration of free sulphuric acid in the wash circuit and decreases the quantity of solids produced in a raw liquor neutralization step, which in turn limits soluble losses in the wash circuit. Although the same quantity of gypsum solids are produced with or without the slurry neutralization step, the gypsum solids in the slurry neutralization step are produced at higher temperature and have better liquid-solid separation characteristics. Sulphide precipitation is also successfully carried out under milder conditions than at Moa. The pH in the raw liquor neutralization step is maintained at about pH 2.5, hence contamination of the mixed sulphide with iron and silicon remains an issue. The thickener underflow slurry in the raw liquor neutralization step contains 20% solids at best. A portion of the thickener underflow slurry, enriched in the fines fraction following treatment in a hydrocyclone, is recycled to the raw liquor neutralization circuit; the remainder is directed to the CCD wash circuit. This complicated means of dealing with the raw liquor neutralization thickener underflow solids is necessitated by the poor liquid-solids separation characteristics of the slurry produced in the circuit.
A third acid leach/sulphide precipitation process plant recently commissioned in the Philippines is disclosed in a paper by Tsuchida et al. in 2004 entitled “Development of Process Design for Coral Bay Nickel Project”. Preneutralization of autoclave discharge slurry prior to liquid-solid separation, further neutralization of the solution in a second neutralization with limestone, washing of this second neutralization precipitate in a CCD wash circuit and recycle of barren liquor to the wash circuit are practiced, with sulphide precipitation carried out under relatively mild conditions. A zinc removal step is included, using hydrogen sulphide gas to selectively precipitate zinc sulphide, before precipitation of nickel and cobalt sulphides. This separate zinc removal step is included so that the product mixed sulphide is substantially free of zinc, a requirement of the subsequent refining step in Japan. The wash circuit product solution is neutralized to pH 3.2 in the raw liquor neutralization circuit. Silicon remains in solution at this pH. Barren liquor is returned to the wash circuit without prior neutralization.
The design of a fourth application of laterite processing including precipitation of mixed sulphide intermediate has also been reported in a paper presented by Faris et al. in 1997 entitled “The Calliope Project: Pressure acid leaching of nickel laterite ores from New Caledonia”. In this process, which was never commercialized, the ore leach discharge slurry from a pressure acid leach is thickened and washed and the solution is subjected to a reduction, a portion of the reduced solution is subjected to an atmospheric acid leach with a portion of the fresh ore, and the atmospheric leach slurry is thickened prior to neutralization of the combined product liquor and precipitation of mixed sulphides. Recycle of a portion of the raw liquor neutralization thickener underflow slurry to the raw liquor neutralization circuit is applied to increase the solids content of the thickener underflow slurry. In so doing, less of the concentrated nickel and cobalt bearing solution is directed to the wash circuit, improving overall nickel and cobalt recovery. Barren liquor from sulphide precipitation is neutralized with limestone and used as wash liquor in the CCD circuit.
The refining of mixed sulphide to nickel and cobalt metals is described in the Motteram et al. paper discussed above. The mixed sulphide is first solubilized in an oxidizing pressure leach. Iron is removed from the leach solution by pH adjustment with ammonia. Copper and zinc are precipitated as their sulphides by contact with hydrogen sulphide gas. The purified solution proceeds to solvent extraction, where cobalt is separated from nickel and these metals are precipitated as metal powders by reduction with hydrogen. Following stripping of residual metals in the nickel and cobalt reduction end solutions with hydrogen sulphide, ammonium sulphate is recovered from the process liquor by evaporation and crystallization.